A 35-year-old man with no prior cardiac history presents with a 6-hour history of palpitations. He denies chest pain or dyspnea. On examination, BP is 132/82 mmHg, HR 160/min, regular. The JVP shows regular oscillations without cannon a waves. A 12-lead ECG reveals a narrow-complex tachycardia at 160/min; P waves are not clearly visible in the surface ECG. Carotid sinus massage is performed, which transiently slows the AV nodal conduction but does not terminate the arrhythmia; the tachycardia resumes at 160/min. Adenosine 6 mg IV is then administered, resulting in transient AV block and brief sinus rhythm, after which the tachycardia resumes at the same rate. What is the most likely diagnosis?